1. Field of the Invention
The present invention generally relates to control systems in vehicles such as motorcycles, ATVs (all terrain vehicles), or small trucks. More particularly, the present invention relates to control of driving power sources to improve drivers' feeling during, for example, starting of the vehicles.
2. Description of the Related Art
In general, a vehicle drives on a bad road surface or rough road surface of a gravel road, a muddy road, a sandy road, etc., as well as a good road surface of a dried pavement, etc. During driving on the bad road surface, a wheel's (or tire's) grip force on the road surface tends to be small. If an engine speed increases in excess during starting, the drive wheel slips, and an impact is transmitted to a driver at a time point when the drive wheel's grip force on the road surface is restored after the slip is mitigated. This may result in a situation where the vehicle cannot start smoothly.
In addition to the above case, if a clutch is operated unskillfully by a driver in a relatively high engine speed condition during driving at a low speed, a driving power transmitted to the wheel changes rapidly, and the resulting impact is transmitted to the driver. Besides, due to such a rapid change in the driving power, the drive wheel slips for a moment and the resulting impact is transmitted to the driver as in the above case. Under such a high engine speed condition, the vehicle cannot start and accelerate smoothly.
Under the circumstances, to suppress a drive wheel slip, traction control is used, in which a slip state of the drive wheel is detected based on a signal from a rotational speed sensor attached on the drive wheel and an engine torque is reduced as necessary based on the detected slip state. The slip of the drive wheel can be suppressed by reducing the engine torque.
Japanese Laid-Open Patent Application Publication No. 2006-329118 discloses a leisure vehicle in which a rotational speed of a front wheel which is a driven wheel is assumed as a vehicle speed and an engine speed is controlled to be a predetermined engine speed (e.g., about 700 rpm) irrespective of the driver's accelerator operation, when this vehicle speed is zero, a transmission gear position is a first gear position, and the engine speed is greater than or equal to a predetermined value, i.e., during starting of the vehicle, thereby enabling the vehicle to start smoothly even on the bad road surface.
However, in the conventional traction control generally used, it is necessary to perform feed-back control of the output of the engine speed sensor, and such control is complex. After the traction control is initiated, an engine torque continues to be suppressed irrespective of the driver's accelerator operation, during a period of time until the slip of the drive wheel finishes and the traction control is terminated. During the period of time, the driver's operation of an accelerator or the like is not reflected by the vehicle's behavior. In addition, in the traction control, since the engine torque is reduced after the slip of the drive wheel has occurred, the impact cannot be mitigated sufficiently sometimes, which leaves room for improvement.
In the case where the engine speed is controlled irrespective of the driver's accelerator operation during starting, as disclosed in Japanese Laid-Open Patent Application Publication No. 2006-329118, the driver feels discomfort because the accelerator operation is not reflected well by the vehicle's behavior. In the conventional example, since the engine speed is set considerably low to enable the vehicle to start smoothly even on the bad road surface or the like, the driver feels that the engine speed does not substantially increase regardless of the driver's operation of the accelerator or the like, and an engine stall could occur if the clutch is operated unskillfully by the driver.